1. Field of the Invention
The present invention relates generally to an electronic flash apparatus of a camera and, more particularly, to a camera electronic flash apparatus so employed as to be mounted in a camera incorporating a focal plane shutter.
2. Related Background Art
Typically, a ready light is installed in an electronic flash apparatus so employed as to be mounted in a camera incorporating a focal plane shutter. The ready light for indicating a charge level of a main capacitor for storing a flash energy is installed to indicate a state where the electronic flash apparatus can flash.
This ready light is generally adjusted to be energized at an energizing voltage with an underexposure of approximately 1 [EV] as compared with the flash when the main capacitor is full of electric charge.
Hence, if a flashtube is fired immediately after energizing the ready light, a photo shot with a full-open exposure of the focal plane shutter is subjected to the underexposure of about 1 [EV] on the entire picture at the worst case.
In this case, an aperture of the photographing lens is opened by one f-number when taking the photo, thus making it possible to provide the photo taken with a proper exposure.
Contrastingly, in the case of photography at a higher shutter speed, the shutter provides not the full-open exposure but a slit exposure. The whole film surface can not be exposed by a single flash. A practice is therefore that the entire film surface is exposed by continuing a high-speed repetitive flash (FP flash) during the slit exposure of the slit surface.
There arise, however, the following problems inherent in the conventional electronic flash apparatus. The underexposure tends to be caused in the whole photographed picture by the flash immediately after or before energizing the ready light in the single flash. Contrastingly, as illustrated in FIG. 5, an intensive underexposure indicated by oblique lines is produced in only one portion on the photographed picture by the flash immediately after or before energizing the ready light in the high-speed repetitive flash. This results in a photo containing a vignetted portion K.
More specifically, the high-speed repetitive flash is intended to expose the film surface with the light. This exposure involves a generation of approximately uniform light in by repeating the flash of a short duration at a high speed and irradiating a slit formed between a shutter leading curtain C and a shutter trailing curtain D with the light. Besides, a flash quantity for each operation is predetermined in the high-speed repetitive flash. Therefore, before a voltage of the main capacitor comes to a predetermined voltage, i.e., when effecting the high-speed repetitive flash in a state where the voltage of the main capacitor is insufficient, as shown in FIG. 4, the flash is terminated at a timing t.sub.6. The photo turns out as shown in FIG. 5.
Note that the flash quantity per operation may be set somewhat small for preventing this state. The high-speed repetitive flash is, however, very small in the flash guide number in terms of characteristics thereof. Taking account of a condition in use, it is therefore desirable to the user that the flash quantity is set to, even if small, increment the flash guide number.
Further, so-called stroboscopic photography based on a combination of the camera and the electronic flash apparatus is conducted with a single flash to singly fire the flashtube.
To be more specific, as illustrated in FIG. 8, the flashtube is singly fired during a time period (between t.sub.2 -t.sub.4) for which the shutter leading and trailing curtains C, D of the focal plane shutter are fully opened on a film surface F. The film surface F is thereby exposed with a flash waveform G as shown in FIG. 8.
Then, in the case of this single flash, a flash starting timing of the flashtube is a timing t.sub.2 shown in FIG. 8, viz., a timing when the tripping of the shutter leading curtain C on the film surface F is ended.
In the stroboscopic photography based on the single flash described above, however, the shutter curtains have to be invariably fully opened. This naturally involves a limit in terms of shutter flashsync time of second in the stroboscopic photography.
On the other hand, there have recently been developed a so-called high-speed repetitive flash wherein the flashtube is fired repeatedly at a high speed as depicted in FIG. 7, and the flash with a flash waveform H is thereby made consecutive for a duration well longer than in the single flash. The stroboscopic photography based on this high-speed repetitive flash is adopted for a slit flashsync at a higher shutter speed.
An output timing when this high-speed repetitive flash is started is a timing t.sub.2 shown in FIG. 7, i.e., a timing when starting an exposure of the film surface F (a traverse point over A in FIG. 7) after the shutter leading curtain C has been tripped.
Then, the film surface F is, as illustrated in FIG. 9, exposed slitwise sequentially from a side A to a side B at the slit flashsync for a duration of this high-speed repetitive flash.
Generally, however, there is some delay with respect to the start of flashing of the flashtube till a flash intensity of the flashtube becomes substantially uniform in the high-speed repetitive flash. This leads to a problem of producing a so-called vignetted portion K where a start edge of an exposure frame on the film surface F is, as indicated by oblique lines in FIG. 10, not exposed at the slit flashsync due to this delay.
Then, this vignetted portion K becomes larger with a higher shutter speed of second which involves a narrower shutter slit width.